Good 'Code' sequence, then bad

I'm thinking a lazy earthing point somewhere - not sure of most likely position ?

I went to start one of my 20VT's the other day, which has been mothballed for over 10 years and has been starting fine many times over that long period, but I couldnt start it on that day (or today)

Its the CODE light being the pointer to all this.

Basics :- Battery fine, fuel pump pumping fine, engine turns absolutely fine.

The issue:-

Key to Ignition on
CODE light on briefly, then off (normal, expected, all good)

Key to Start (energise starter motor, motor turns engine)
CODE light comes on straight after starter motor turns
And therefore ignition sequence disabled.

Repeat process, same result (good CODE function at ignition position, errors on engine starter rotation)


I am erring towards a bad earth 'somewhere' being the cause of this undocumented feature, pulling down the volts across the ECU and therefore locking it out.

Anyone had any similar experiences, or know the answer ?

Thanks

If the code light comes on and goes off after a second (as normal) then the engine ECU is happy with the code system (valid key detected) and the immobiliser is disabled, doesn't matter what happens with the code box after that until the ignition is switched off again. You can even unplug the code box completely once the code light has gone out and engine will start as normal, so I don't think you have an immobiliser problem unless the code box is crashing and switching off the light itself in which case the injection light would come on as the engine ECU wouldn't receive the expected commands from the code box.

Is injection light coming on as normal then going off and staying off during cranking or is it coming back on?

Can you hear relays clicking in passenger footwell during cranking? I wonder if you have a power/battery problem and cranking the starter is causing voltage to drop making the code box and engine ECU reset/reboot , switching the code light back on again and re-enabling the immobiliser in the ECU. You may see injection light coming back on in that case.

A long, slow battery discharge has been known to permanently corrupt the code box, causing some or all keys to no longer be recognised. Did you try with all available keys?

You can also try disconnecting battery for a minute to reset everything and pulling out and re-inserting both plugs at the code box itself (yellow box in dash above fuse box in driver's footwell), sometimes they can have a dodgy connection there.